Imaging apparatus including three laser diodes and a photographic element

ABSTRACT

An apparatus for producing a full color image including a photosensitive element having three silver halide emulsion layers, at least two of which are sensitized to radiation in the infrared region of the electromagnetic region, and three laser diodes for exposing the photosensitive element. The three silver halide emulsion layers are differentiated from each other by photographic speed, filter layers between the emulsion layers or a combination of both speed differences and filter layers.

This is a continuation of application Ser. No. 736,252 filed 22 May1985, U.S. Pat. No. 4,821,113.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and method for providing colorphotographic images, and particularly such images produced from adigital or analog data base utilizing a laser radiation source.

2. Description of the Prior Art

U.S. Pat. No. 4,346,401 discloses apparatus and a method for producing afull color, continuous tone image on a color film utilizing threegaseous lasers, each laser emitting light in the visible region of theelectromagnetic spectrum corresponding to one of the three primarycolors. Any color of the visible portion of the electromagnetic spectrummay be obtained on the film, upon development, and continuous toning ofthe colors is produced by modulating the output of the gaseous lasers tovary the intensity of the light emitted by the lasers and thereby theexposure of the color film and the density of the colors produced in thefilm upon development.

This system is effective to produce color images but has three majordrawbacks. First, a helium-neon laser, an argon laser and ahelium-cadmium laser must be provided, each of which is relativelyexpensive, has a relatively short life and requires special care andhandling. Second, an external modulator must be provided for each lasersince the intensity of the light emitted by each laser cannot bedirectly modulated. External modulators are also expensive and multiplythe cost of the system. Third, conventional color film is utilized andthus the imaging operation must be performed in visible light-proofconditions to avoid exposure of the film. Additionally, special darkrooms must be provided for the imaging operation and control overingress and egress to the room must be maintained to prevent inadvertentadmission of outside light.

U.S. Pat. No. 4,416,522 discloses apparatus and a method whicheliminates the disadvantages associated with the use of conventionalcolor film. This patent utilizes a color film which produces visiblecolors upon exposure to radiation in the non-visible portion of thespectrum. Thus, the resulting dye colors in the developed film belong toa spectrum (visible) different from the spectrum (non-visible) whichexposes the film. With such a film, one can select exposing radiation inthe various portions of the non-visible spectrum and expose the film indaylight to produce color images in the visible portion of the spectrumwhen the film is developed. The film is exposed by a lamp which emits abroadband beam including multiple wavelengths of non-visible radiantenergy to which the emulsion layers of the film are variously sensitive.Exposure of the film to a particular wavelength or radiation isaccomplished by interposing a filter between the lamp and the film whichfilters out all radiation except that at the desired wavelength.Variations in the intensity of the radiation beam produced by the lampto vary the exposure of the film and the density of the color producedis achieved by increasing or decreasing the speed of the film relativeto the exposing beam and thus changing the duration of contact betweenthe beam and a particular portion of the film.

Two major disadvantages are associated with this system. First, changinga filter to produce individual colors is necessarily slow in itself andalso slows down the process because the surface of the film must becovered three times to produce the three primary colors in the film upondevelopment. Second, modulation of the intensity of the exposing beam byincreasing or decreasing the relative speed of the beam and the film isexpensive since servo motors and their associated controls must beprovided for each of the x and y coordinates of the film surface.

With respect to photosensitive films, dyes which have been capable ofsensitizing silver halide emulsions to infrared region os theelectromagnetic spectrum have been known for many years. Merocyaninedyes and cyanine dyes, particularly those with longer bridging groupsbetween cyclic moieties have been used for many years to sensitizesilver halide to the infrared. U.S. Pat. Nos. 3,619,154, 3,682,630,2,895,955, 3,482,978, 3,758,461 and 2,734,900; and U.K. Patent Nos.1,192,234 and 1,188,784 disclose well-known classes of dyes whichsensitize silver halide to portions of the infrared region of theelectromagnetic spectrum. U.S. Pat. No. 4,362,800 discloses dyes used tosensitize inorganic photoconductors to the infrared, and these dyes arealso effective sensitizers for silver halide.

With the advent of lasers, and particularly solid state laser diodesemitting in the infrared region of the electromagnetic spectrum (e.g.780 to 1500 nm), the interest in infrared sensitization has greatlyincreased. Many different processes and articles useful with laserdiodes have been proposed. U.S. Pat. No. 4,416,522, for example,proposes daylight photoplotting apparatus for the infrared exposure offilm. This patent also generally proposes a film comprising threeemulsion layers sensitized to different portion of non-visible portionsof the electromagnetic spectrum including the infrared. The filmdescription is quite general and the concentration of imagewise exposureon each layer appears to be dependent upon filtering of radiation by theapparatus prior to its striking the film surface.

SUMMARY OF THE INVENTION

The present invention eliminates the high inherent cost and expense ofexternal modulation associated with gaseous lasers by providing a laserdiode imaging apparatus which produces multi-color, continuous toneimages on a photographic element upon exposure to laser radiation.

A photographic element is described which is capable of providing fullcolor images without exposure to corresponding visible radiation. Theelement comprises at least three silver halide emulsion layers on asubstrate. The at least three emulsion layers are each associated withdifferent photographic color image forming materials, such as colorcouplers capable of forming dyes of different colors upon reaction withan oxidized color photographic developer, diffusing dyes, bleachabledyes, or oxidizable leuco dyes. The three emulsion layers are sensitizedto three different portions of the electromagnetic spectrum with atleast two layers sensitized to different regions of the infrared regionof the electromagnetic spectrum. The layers must be in a constructionthat prevents or reduces the exposure of layers by radiation intended toexpose only one other layer. This is done by providing differences inspeed of emulsion sensitive to different wavelengths of the infrared,and/or by providing filters between the layers.

The apparatus for producing full color, continuous tone images includeslaser diodes corresponding in number to the number of the photosensitivelayers of the photographic element, with each laser diode emittingradiation at a different wavelength and corresponding in wavelength tothe wavelength at which a different photosensitive layer is sensitized;directing radiation from the laser diodes to the photographic element sothat radiation from each of the laser diodes exposes each of thephotosensitive layers to produce a color in each photosensitive layerupon reaction with a photographic developer; and modulating continuouslythe intensity of the radiation emitted by each of the laser diodes anddirected to the photographic element so that the density of the colorproduced upon reaction with a photographic developer is correspondinglymodulated in a continuous tone. The apparatus may further include meansfor combining the radiation emitted by the laser diodes into a singlebeam and traversing the photographic element with the radiation emittedby the laser diodes and simultaneously moving the photographic elementin a direction having a component direction perpendicular to thedirection of traversal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more thoroughly described with referenceto the accompanying drawings wherein like numbers refer to like parts inthe several views, and wherein:

FIGS. 1A and 1B are schematic views of a laser diode imaging apparatusaccording to the present invention;

FIG. 2 is a schematic view of the laser diode imaging apparatus of FIG.1 viewed generally from the perspective of line 2--2 of FIG. 1;

FIG. 3 is a cross-sectional view of a photographic element used inconjunction with the laser diode imaging apparatus of FIG. 1;

FIG. 4 is a schematic perspective view of an alternate scanning methodwhich may be employed with the laser diode imaging apparatus of FIG. 1;and

FIG. 5 is a schematic perspective view of a second alternate scanningmethod which may be employed with the laser diode imaging apparatus ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Briefly stated, and with reference to FIG. 1, the present inventionincludes a laser diode imaging apparatus generally indicated as 10 whichincludes three laser diodes 12, 12a and 12b which emit radiation in theinfrared region of the electromagnetic spectrum and preferably atwavelengths of 780 nm, 830 nm and 880 nm. This infrared radiation iscombined into a single beam 14 which is scanned across a photographicelement receptor surface 16 by means of a rotating or oscillating singleor multi-surface mirror 18.

In the preferred embodiment, the photographic element 16 is capable ofproducing a visible, full color and continuous tone image upon exposureto radiation in the infrared region in the electromagnetic spectrum,without exposure to radiation or light contained within the visibleregion of the electromagnetic spectrum. This is accomplished byconstructing the photographic element 16 of at least threephotosensitive layers which correspond in number to the number of laserdiodes 12, 12a and 12b. Each photosensitive layer is capable ofproducing one of the additive primary colors (red, blue or green) or oneof the subtractive primary colors (cyan, magenta or yellow) uponexposure to infrared radiation corresponding to the radiation emitted byone of the laser diodes 12, 12a or 12b.

A full color image is produced by superposition of the at least threephotosensitive layers and, therefore, the superposition of the additiveprimary or subtractive primary colors produced upon exposure anddevelopment. Thus various color may be produced by the relative exposureof each of the photosensitive layers and the relative presence orabsence of one or more of the additive primary or subtractive primarycolors. A continuous tone image is produced by controlling the overalldensity of the superposed colors produced upon development, and thiscolor density may be controlled by simultaneous independent modulationof the intensity of radiation produced by the laser diodes 12, 12a and12b and thus modulation of the total exposure of all photosensitivelayers.

The Photographic Element

FIG. 3 is an enlarged, cross-sectional view of a portion of thepreferred photographic element 16 which includes a support 20, which ispreferably a standard resin-coated photographic grade paper but whichmay be a clear polyethylene terephthalate film base, a firstphotosensitive emulsion layer 22 sensitized to a radiation wavelength of880 nm, a first gelatin interlayer 24, a second photosensitive emulsionlayer 26 sensitized to radiation at a wavelength of 830 nm, a secondgelatin interlayer 28, a third photosensitive emulsion layer 30sensitized to radiation at a wavelength of 780 nm, a third gelatininterlayer 32 and a protective gelatin topcoat 34.

The emulsion layers 22, 26 and 30 may be any of the various types ofphotographic silver halide emulsions commonly used such as silverchloride, silver bromide, silver iodobromide, silver chlorobromide,silver chlorobromoiodide and mixtures thereof.

In order that each emulsion layer 22, 26 or 30 be sensitized to respondto its chosen specific wavelength of the infrared spectrum, thesensitizing dyes chosen are extremely important. A large number of dyesare known to sensitize silver halide emulsions to various portions ofthe infrared region of the electromagnetic spectrum. Cyanines andmerocyanines are well documented as infrared sensitizers for varioustypes of imaging systems including silver halide emulsions, with theusual dye structures being symmetrically or unsymmetrically substituteddicarbocyanines and tricarbocyanines with the auxochromic portions ofthe dyes being lepidine, quinoline, naphthothiazole, or benzothiazole.Heterocyclics may also be introduced to increase the rigidity andstability of the dye molecules.

In addition, each emulsion layer 22, 26 or 30 must contain a differentcolor photographic coupler which is capable of forming a different colordye upon development by reaction with an oxidized color photographicdeveloper. The dye-forming couplers are commonly chosen to form one ofthe three subtractive primary colors (yellow, magenta and cyan) in eachof the three emulsion layers 22, 26 and 30. Most useful arenondiffusable, colorless couplers which may be chosen from the variousclasses of B-keto-carboxamides (yellow couplers), 1-aryl-5-pyrazolones(magenta couplers) and either phenols or naphthols (cyan couplers).

As stated above, the interlayers and protective topcoat layer 24, 28, 32and 34, preferably are gelatin but other hydrophilic or hydrophobicbinders may be used, providing adequate physical characteristics such ashardness and defusability are maintained. The gelatin layers 24, 28, 32and 34 may also contain hardners, U.S. absorbers and antioxidents as iswell known in the art.

The photographic element 16 thus far described would be completelyadequate to the practice of the present invention if the sensitizingdyes utilized in the emulsion layers 22, 26 and 30 had essentiallymonochromatic absorption curves which corresponded to the monochromaticradiation emitted by the laser diodes 12, 12a and 12b. Unfortunately,while the individual sensitizing dyes contained in the emulsion layers22, 26 and 30 may be selected to have maximum sensitivity at wavelengthscorresponding to those emitted by the laser diodes 12, 12a and 12b,these sensitizing dyes have a range of absorption which may extend froma few nanometers up to a few hundred nanometers from the wavelength ofmaximum sensitivity. The characteristic shape of absorption curvescorresponding to the sensitizing dyes listed above shows a broad tail ofsensitization stretching 150 to 300 nanometers from the peak of maximumsensitization toward the shorter wavelength side of the electromagneticspectrum and a narrower tail of sensitization extending approximately 50to 70 nanometers wide toward the longer wavelength side of theelectromagnetic spectrum. Sensitizing dyes other than the particulardyes listed are known to exhibit similar absorption curves.

State of the art laser diodes typically emit radiation betweenwavelengths of 750 to 950 nanometers and, as stated above, theparticular laser diodes 12, 12a and 12b used in the imaging apparatus 10have been selected to emit radiation at about 780, 830 and 880nanometers. This separation of only 50 nanometers between the radiationemitted by successive laser diodes tends to be too narrow a range toallow for multiple layer photographic emulsions 22, 26 and 30 withtotally different regions of sensitivity. Although each of thesensitizing dyes contained in one of the emulsion layers 22, 26 or 30can be selected to correspond fairly precisely to the radiation emittedby any of the laser diodes 12, 12a or 12b, the absorption curves for thesensitizing dyes dictate sensitizing effects which could overlap intothe wavelengths emitted by the remaining laser diodes. Particularly in aphotographic element 16 intended to provide a full color image, thisoverlap of sensitivity causes poor faithfulness in color renditionbecause of spurious imaging of the multiple layers 22, 26 and 30 by asingle wavelength of radiation.

In order to combat the formation of spurious images, the emulsion layers22, 26 and 30 are laid down in a particular order with regard to theirrespective sensitivities. The emulsion layer containing sensitizing dyessensitive to the shortest wavelength are located within the emulsionlayer 30 farthest from the substrate 20 and the emulsion layercontaining the dyes sensitized to the longest wavelengths are locatedwithin the emulsion layer 22 located closest to the substrate 20.Spurious images created by the exposure of emulsion layers 22, 26 or 30by radiation intended to expose only one other layer is also preventedor reduced by increasing the difference in the speed of the emulsionlayers 22, 26 or 30 sensitive to different wavelengths of the infrared,providing infrared radiation filters within the gelatin interlayers 24,28 or 32 that absorb ranges of infrared radiation, or combining bothdifferential speeds and filters within the single photographic element16.

If it is required to reduce or eliminate spurious images by differentialspeeds between the emulsion layers 22, 26 and 30 and not rely onfiltering layers, it is required that each of the three emulsion layers22, 26 and 30 have a contrast between two and eight and differ from eachother in photographic speed such that, at an optical density of 1.3, thespeed of emulsion layer 30 is at least 0.2 log E units faster than theemulsion layer 26 and the speed of the emulsion layer 26 is at least 0.2log E units faster than the emulsion layer 22.

The higher the contrast in the emulsion layers 22, 26 and 30 of thepresent invention, the smaller need be the differences in speed. Forexample, with a contrast of eight for the emulsion layers 22, 26 and 30,a speed difference of 0.2 log E units at their wavelengths of maximumsensitivity would be sufficient. Below about 4.5 in contrast, however,the difference in speed must be at least 0.4 log E units, and with acontrast between about two and four, the speed difference must be atleast 0.5 log E units.

If it is desired to eliminate spurious images by the use of filterlayers, without regard for the speed of the emulsion layers, it isrequired that a filter layer be located between emulsion layer 22 andemulsion layer 26 which absorbs infrared radiation in a rangeoverlapping the region of maximum sensitivity of the emulsion layer 26without absorbing more than 40% of the infrared radiation to which theemulsion layer 22 is sensitized, and that a filter layer be locatedbetween the emulsion layer 26 and the emulsion layer 30 which absorbsradiation in a range overlapping the region of maximum sensitivity ofthe emulsion layer 30 without absorbing more than 40% of the infraredradiation to which the emulsion layer 26 is sensitized.

Filter dyes and their methods of incorporation into photographicelements are well documented in the literature. If such dyes are used,they must be selected on the basis of their radiation filteringcharacteristics to ensure that they filter the appopriate wavelengthsand should also be provided with non-fugitive characteristics and shouldbe decolorizable or leachable.

These two methods of reducing or preventing spurious images may becombined in one photographic element 16 by incorporating a filter dyebetween one of the pairs of adjacent emulsion layers 22 and 26 or 26 and30 and regulating the contrast and speed between the remaining pair ofadjacent emulsion layers.

Further details of the elements included in and the construction of thepreferred photographic element 16, and alternative constructions, may beobtained with reference to assignee's copending U.S. patent applicationSer. No. 709,561 to Simpson, filed Mar. 8, 1985, now U.S. Pat. No.4,619,892, issued Oct. 28, 1986.

Laser Diodes and Focusing System

Referring again to FIG. 1, there is schematically illustrated a laserdiode imaging apparatus 10 which includes three laser diodes 12, 12a and12b. Associated with each of these laser diodes 12, 12a and 12b arevarious elements used to regulate, focus and combine the radiationemitted by each of the laser diodes 12, 12a and 12b. Since the laserdiodes 12, 12a and 12b and their associated elements operateidentically, reference hereafter will be to only one of the triplicatedelements for brevity and clarity of explanation. It is understood,however, that reference to and description of one element includes anddescribes its triplicate counterparts. For example, future reference tolaser diode 12 will also include laser diode 12a and laser diode 12b.

Laser diodes 12 useful in the present invention have a power capabilityof at least 3 milliwatts and preferably 15 to 30 milliwatts, and adynamic range of at least 20:1 and preferably 30:1. Radiation emitted bythe diodes 12 is preferably in the infrared region of theelectromagnetic spectrum between about 750 and 900 nanometers. Laserdiodes 12 meeting the above criteria are commercially available fromHitachi (Tokyo, Japan), Mitshubishi (Tokyo, Japan), RCA (Lancaster, PA),Sharp (Osaka, Japan) and Philips (Eindhoven, Netherlands).

In order to be useful, the radiation emitted by the laser diodes 12 mustbe capable of being modulated between lower and higher values in orderto vary the exposure of the photographic element 16, described above,and thereby the color density of the photosensitive layers 22, 26 and 30comprising the photographic element 16. Modulation of the radiationemitted by the laser diodes 12 is accomplished by varying the forwardbiasing current 36 supplied to the laser diode 12 by a driving circuit38. The driving circuit 38 is responsive to three inputs which aresummed together by the driving circuit 38 and which determine the valueof the forward biasing current 36 supplied to the laser diode 12.

One of the signals supplied to the driving circuit 38 is an analogsignal 40 supplied directly by an analog video data source or by adigital-to-analog converter 42 such as Model No. AH8308T made byAnalogic Corporation, Wickfield, MA. The purpose of thedigital-to-analog converter 42 is to convert digital data containingpicture information into the analog signal 40 which may be utilized bythe driving circuit 38.

Analog data may be supplied, for example, from a video camera or videodisplay system which is appropriately synchronized with the laser diodeimaging apparatus or digital data may be supplied from medical imagingsystems, weather or military satellites, video cameras, opticaldigitizers or computer memory in which an image is stored as a number ofpicture elements or pixels as is well known in the art. This digitaldata may also be stored in random access memory magnetic disks, opticaldisks and the like.

The second input 44 to the driving circuit 38 is supplied by a motorcontroller 46 which is associated with the rotating or oscillatingsingle or multi-surfaced mirror 18. Preferably, the mirror 18 is amulti-faceted polygonal mirror which rotates to scan the combined beam14 of the laser diodes 12 across the photographic element 16. Asignificant problem associated with a polygonal mirror 18 isreflectivity variations between different facets of the polygon. Ifuncorrected, these reflectivity variations would result in a change inthe intensity of the combined radiation beam 14 and, consequently, achange in the exposure of the photographic element 16. To combat thisproblem, the motor controller 46 is provided with a facet countingcapability and reflectivity information concerning the various faces ofthe rotating mirror 18. This information is manifested in the inputsignal 44 which causes the driving circuit 38 to increase or decreasethe intensity of radiation emitted by the laser diode 12 depending uponthe reflectivity of the particular mirror facet presented to theradiation beam 14. Of course, if only a single facet is utilized, or thevariation between facets is insignificant, this second input 44 may beeliminated.

The third input 48 to the driving circuit 38 is produced by an infraredphotoelectric cell 50 which is responsive to a fractional beam 52derived from the output beam 54 of the laser diode 12. The fractionalbeam 52 is produced by means of a beam splitter 56 which may be achrome-type neutral density filter such as those produced by MellesGriot of Irving, CA or a cube beam splitter. The infrared photoelectriccell 50 is necessary because semiconductor laser diodes 12 exhibitvariations in their power output at a constant biasing current level dueto the effects of temperature, aging and the phenomenon of mode hopping,which is the switching from one resonant mode to another within thelasing cavity of the laser diode 12 and which results in slightlychanging wavelengths and lasing efficiencies. In order to correct forthese effects and to maintain the stability of the radiation emitted bythe laser diode 12, a feedback system is employed.

In this feedback system, the infrared photoelectric cell 50 continuouslymonitors the output of the laser diode 12 and produces the input signal48 which adjusts the laser diode 12 to stabilize its operation. Thisfeedback system allows virtually instantaneous changes in the outputintensity of the laser diode 12 in response to the fractional beam 52and is capable of maintaining the intensity of output beam 54 of thelaser diode 12 constant regardless of long or short term changes in theradiation intensity versus biasing current characteristics of the laserdiode 12.

Techniques and circuitry to accomplish this type of continuous feedbackcontrol are disclosed by M. Lutz, B. Reiner, and H. P. Vollmer in"Modulated Light Source for Recording With GaAlAs-Lasers", presented atFirst International Congress on Advances in Non-Impact PrintingTechnology, Venice, Italy (July 22-26, 1983) and D. R. Patterson and R.B. Childs in "Semiconductor Lasers Reach for Maturity: Applications inFiber Optic Communications", Photonics Spectra, pages 83-87, (April1982).

By means of the three inputs 40, 44 and 48 to the driving circuit 38 theoutput beam 54 of the laser diode 12 is constantly modulated, correctedfor reflectivity variations of the rotating mirror 18 and corrected forvariations in the operating characteristics of the laser diode 12itself. Thus, by means of the two corrective inputs 44 and 48, veryprecise control of the diode 12 output is achieved.

The output beam 54 which is emitted from the laser diode 12 possessescharacteristics which the optical system of the imaging apparatus 10must accommodate. Laser diodes 12 of the type described herein producean output beam 54 which is divergent and, in addition, may be highlyastigmatic in that the component of the beam parallel to thesemiconductor junction appears to originate from a source at a differentlocation along the cavity than the component of the beam 54perpendicular to the junction. To correct for these characteristics ofthe output beam 54 the imaging apparatus 10 is provided with acollimating lens 58 which corrects the beam 54 from a divergent beam toa beam having parallel rays and a cylindrical lens system 60 whichcorrects the beam's 54 astigmatism and cause the beam 54 to becomecircular in cross-section.

After the cylindrical lens system 60, the beam 54 passes through aneutral density attenuator 62 which matches the output of the laserdiode 12 to the characteristics of the photographic element 16. Sincethe lenses 58 and 60 and the attenuator 62 change the characteristics ofthe output beam 54, it may be advantageous to interpose the beamsplitter 56 between one or all of these elements and the laser diode 12so that the photoelectric cell 50 is struck by a beam 52 which is morerepresentative of that actually emitted by the diode 12.

The beams 54, 54a and 54b emitted respectively from the laser diodes 12,12a and 12b are combined by beam combiners 64 and 66 (which may beidentical to the beam splitter 56) into the single beam 14 which is aresulting three-wavelength beam containing three preferably colinear butindependent beams. Three spaced, adjacent beams may also be utilized, ifdesired.

These combined beams, herein referred to as a single beam 14, are thendirected toward the facets of the rotating polygonal mirror 18 through acylindrical lens system 67. Scanning, which is the continual sweeping ofthe radiation beam 14 across the surface of the photographic element 16,preferably is achieved by the high speed rotation of the polygonalmirror 18 which has one or more reflecting facets. A lesser number ofreflective facets will either reduce the correctional demands placed onthe motor controller 46 or the expense of providing highly uniformeffective surfaces, while a greater number of facets will decrease thetime interval between successively scanned beams and, therefore,decrease the length of time necessary to completely cover the surface ofthe photographic element 16. Although a rotating polygonal mirrror isillustrated and preferred, a scanning galvonometer, acousto-opticdeflector or a holographic deflector, all of which devices are wellknown in the art, may be employed.

As the polygonal mirror 18 rotates in a clockwise direction from theperspective of FIG. 1, the scanned radiation beam 14 moves in a downwarddirection, with respect to FIG. 1, and is reflected by a planar mirror68 to the photographic element 16. The mirror 68 is provided to fold thepath of the radiation beam 14 and thus conserve space, but may beeliminated if space consideration is not a concern. Prior to contactingthe mirror 68, the radiation beam 14 is first reflected to strike astart-of-scan photoelectric detector 70 along the radiation path 72indicated by a phantom line. The detector 70 generates a timing signalwhich causes the driving circuit 38 to initiate modulation and thusinformation transmission to the radiation beam at the proper instant.Thus, at the beginning of each line, a correction for any timing variousduring scanning of the previous line is provided.

Between the rotating mirror 18 and the planar mirror 568 are located twolenses 74 and 76 which correct for problems associated with the use ofthe rotating mirror 18 and scanning of the radiation beam 14. Lens 74 isa cylindrical toroidal lens which, when combined with the cylindricallens system 67, is utilized to correct for an effect known as pyramidalerror introduced by variations in the angles of the planes of the facetsof the mirror 18 with respect to the axis of rotation of the polygonalmirror 18. This pyramidal error is a common problem associated withrotating mirrors and the use of the cylindrical toroidal lens 74 issuggested by U.S. Pat. No. 3,750,189 to Fleischer. The lens 74 providesthat the focus of the beam 14 and the point of minimum pyramidal errorcoincide at the same point on the surface of the photographic element16.

The lens 76 is a so-called "F-θ" lens and is used to maintain thevelocity of the beam 14 across the mirror 68 and the surface of thephotographic element 16 at the constant value. In common imaging lensesthe location of the beam 14 (r) on an imaging plane at a givenprojection angle (θ) is given by the relationship:

    r=f·tan θ

where f=the focal length of the imaging lens.

In such a system the projection angle of the radiation beam on theimaging lens linearly changes with time. Accordingly, the moving speedof the scanned beam on the imaging plane changes non-linearly. With anincrease of projection angle, the velocity of the beam 14 increases. Ifthe scanned beam 14 is considered a series of discrete spots, thisnon-linearity causes the spots to become more spaced apart at the endsof the scan line compared to the spacing of the spots at the center ofthe scan line. In order to avoid this result, the imaging lens 76 in thepresent invention is tailored so as to have the property:

    r=f·θ

A lens with this property is called an f-θ lens and causes the velocityof the scanned beam 14 to be constant across the entire scan line.

The system thus far described provides for the sweeping or scanning ofthe radiation beam 14 in a direction parallel to one edge of thephotographic element 16. Provisions must be made for movement of thescanned radiation beam 14 across the photographic element 16 in adirection perpendicular to its direction of scan. This could beaccomplished by progressively tilting the planar mirror 68 in responseto signals provided by the start-of-scan photoelectric detector 70 orcan be accomplished by moving the photographic element 16 in the mannerillustrated by FIG. 2.

In FIG. 2, the photographic element 16 is supported by a table 78 and isengaged by a driven roller 80 and idler rollers 82 and 84. The drivenroller 80 is controlled by a motor controller 86 which is responsive toa signal provided by the photoelectric detector 70 indicating that thescanning beam 14 is about to begin its sweep of the photographic element16. Just prior to the scanning of the photographic element 16 by thebeam 14, the motor controller 86 incrementally advances the photographicelement 16 so that a new line of the photographic element 16 may beexposed. For simplicity, the photographic element 16 may be drivencontinuously, in which case the detector 70 operates only to ensure thatthe margin of the swept lines is even.

The roller system of FIG. 2 is one possible method of incrementallyadvancing the photographic element 16 but it should be recognized thatmany other systems are possible. For example, the photographic element16 could be attached to a rotating drum, the table 78 could be advancedby a lead screw or the photographic element 16 could form a portion of acontinuous sheet which is advanced relative to the scanning beam 14 bymeans of supply and take-up rolls located on each side of the scanningbeam 14.

As shown in FIG. 4, the motion of the photographic element 16 relativeto the scanned beam 14 need not be entirely perpendicular to thedirection of scan of the beam 14. If the imaging apparatus 10 isarranged such that the scanning beam 14 scans the photographic element16 in a direction which is not parallel to one of the edges of theelement 16, the entire surface of the photographic element 16 may stillbe covered by advancing the element 16 in a direction parallel to one ofits edges. All that is required, therefore, is that the direction ofadvancement of the photographic element contain a component directionwhich is perpendicular to the direction of scan of the scanning beam 14.The only direction of advancement of photographic element 16 which wouldnot be effective is a direction which is entirely parallel to thedirection of scan of the beam 14.

FIG. 5 illustrates yet another method of scanning the radiation beam 14across the entire surface of the photographic element 16. While therotating polygonal mirror 18 is preferable because of the speedassociated with its use, it is possible to insert the photographicelement 16 in the path of the radiation beam 14 between the beamcombiner 66 and the polygonal mirror 18 as these elements areillustrated in FIG. 1. In this instance the photographic element may bemoved in direction both parallel and perpendicular to one of its edgesin the manner described in U.S. Pat. No. 4,416,522 to Webster. In thisevent the polygonal mirror 18, cylindrical toroidal lens 74, f-θ lens 76and planar mirror 68 may be eliminated and the appropriate focusinglenses modified to provide the correct focal point. As illustrated inU.S. Pat. No. 4,416,522, the photographic element 16 may be supported ona table 88 which is moved in perpendicular directions by appropriatelycontrolled driving motors.

Operation

The laser diodes 12, 12a and 12b are selected to emit radiation withinthe infrared region of the electromagnetic spectrum at 780 nm, 830 nmand 880 nm, respectively. These particular values of the wavelength ofradiation emitted by the diodes 12, 12a and 12b are not critical but areselected to provide maximum separation between the wavelengths ofradiation emitted by each of the three laser diodes 12, 12a and 12b.

Maximum separation between the radiation emitted by the diodes 12, 12aand 12b is desirable because this maximum separation will simplify theconstruction of the photographic element 16 since the problem ofspurious images between the photosensitive layers 22, 26 and 30 isreduced with greater wavelength separation. The problem of spuriousimaging between the photosensitive layers 22, 26 and 30 of thephotographic element 16 is discussed above in the section concerning thephotographic element 16.

State of the art laser diodes emit radiation between wavelengths ofapproximately 750-900 nanometers. The selection of laser diodes 12, 12aand 12b emitting radiation at 780, 830 and 880 nm, therefore, representsthe maximum practical spread of wavelengths which can be easily obtainedfrom the range of wavelengths obtainable in commercial laser diodes. Itis contemplated that if laser diodes which emit radiation in otherwavelength regions of the electromagnetic spectrum become available thatlaser diodes will be selected which will have a greater wavelengthseparation than the 50 nm reflected herein.

In preparation for printing an image on the photographic element 16, thelaser diodes 12, 12a and 12b are not receiving any analog input signalfrom the digital-to-analog converter 42 representing information to beimaged, but receive a small current from the feedback circuit, whichincludes the infrared photoelectric cell 50, which maintains the laserdiodes 12, 12a and 12b at their threshold levels so that the laserdiodes will not be turned off completely. The low intensity radiationbeams emitted by the diodes 12, 12a and 12b are optically corrected andcombined, as described above, into a colinear combined beam 14 which isdirected to the rotating polygonal mirror 18. Rotation of the mirror 18causes the reflection of the beam 14 to be swept in a downwarddirection, with reference to FIG. 1, until the beam 14 contacts thestart-of-scan photoelectric detector 70.

The detector 70 provides a signal that the emitted beam 14 is about tobegin its scan of the photographic element 16 and instructs thedigital-to-analog converter to begin transferring information to thelaser diode driving circuits 38, 38a and 38b after a suitable time delaywhich represents the time necessary for the beam 14 to travel from thephotoelectric detector 70 to the photographic element 16 as the beam 14is scanned or swept downwardly.

When the combined beam 14 strikes the photographic element 16, the laserdiodes 12, 12a and 12b are continuously modulated by their respectivedigital-to-analog converters 42, 42a and 42b and driving circuits 38,38a and 38b to regulate the intensity of the radiation emitted by thelaser diodes 12, 12a and 12b in accordance with digital informationsupplied to the digital-to-analog converters 42, 42a and 42b.

The digital information supplied to each of the laser diodes 12, 12a and12b usually corresponds to a different one of the three additive primarycolors (blue, green and red) into which any full-color image may bedivided. The digital information corresponding to the blue constituentof the image may then control laser diode 12 which emits radiation at awavelength of 780 nm. Laser diode 12a, emitting radiation at 830 nm, maycorrespond to digital data representing the color green and the laserdiode 12b, emitting radiation at 880 nm, may be supplied digitalinformation corresponding to the color red. The assignment of particularwavelength to particular colors is purely arbitrary and any laser diode12, 12a or 12b could be used to transmit information relating to any ofthe primary colors, or for that matter, any color.

Each of the three colinear beams from the laser diodes 12, 12a and 12b,which comprise the single beam 14, is modulated constantly andindependently to vary its intensity in accordance with the intensity ofits corresponding color in the image to be printed. The radiationemitted by the laser diodes 12, 12a and 12b may be modulated at afrequency up to the limits of the solid state components of the system,which may be many million samples per second. Typically, however, such ahigh sampling rate will not be used because the human eye cannotdifferentiate such fine detail. The image imparted to the photographicelement 16 will more typically be a series of spots or pixels which willtypically be gaussian or truncated gaussian in shape with a diameter of85 micrometers. The spot diameter my be varied between approximately 5and 1,000 micrometers, depending upon the optical system utilized. Alsotypically, 12 pixels per mm are used to produce the scan lines, although1 to 200 pixels per mm may be utilized, depending upon the resolutiondesired.

By considering one individual pixel, it can be understood how any colormay be produced on the photographic element 16 and how the density ofthis color may be varied. When the combined radiation beam 14 strikes apoint on the surface of the photographic element 16, the radiation fromthe laser diodes 12, 12a and 12b penetrate to expose the threephotosensitive layers 22, 26 and 30. Under rthe hypothetical situationstated above, radiation at 780 nm from diode 12 contains informationcorresponding to the proportion of the color blue in the original imageand so the photosensitive layer 30 will contain dyes which produce thisprimary color when exposed and developed. Likewise, the intermediatelayer 26 responds to the wavelength of 830 nm and produce the colorgreen when exposed and developed. Finally, the lowest emulsion layer 22is particularly sensitive to radiation at 880 nm and will produce thecolor red upon exposure and development. Any given exposed pixel will,therefore, contain the colors blue, green and red superimposed one ontop of the other. When viewed together these three additive primarycolors can produce any color of the visible region of theelectromagnetic spectrum, within the color space defined by theparticular colorants used, by varying the relative density of each ofthe primary colors. The density of each of the colors is controlled bymodulating the intensity of the three colinear beams striking thephotographic element 16. The relative exposure of each of the threephotosensitive layers 22, 26 and 30 will thus determine the colorperceived, and any color within the available color space of thespectrum may be achieved.

To vary the density of any color produced, the intensity of the threebeams produced by the diodes 12, 12a and 12b are modulated in unison butthe relative intensities of the three beams are preserved. Thus, thedensity of any color may be increased by increasing the radiationemitted by all of the laser diodes 12, 12a and 12b while the density ofany color produced on the photographic element 16 may be decreased bysimultaneously limiting the intensity of all of the radiation beamsemitted by the diodes 12, 12a and 12b.

There has been described a laser diode imaging apparatus which canproduce a broad and continuous range of colors of the visible region ofthe electromagnetic spectrum, and at varying densities. Thus, afull-color, continuous-tone image may be produced on a photographicelement.

The foregoing laser diode apparatus has been described with respect to asystem which employs three diodes and a photographic element havingthree corresponding photosensitive layers. This is the most convenientarrangement since this number of elements corresponds to the number ofprimary colors. Any number of diodes and photosensitive layers may beprovided, however, so long as the number of diodes equals the number ofphotosensitive layers comprising the photographic element. For example,it may be advantageous to provide a fourth diode and emulsion layerdedicated to the color black. It is believed that a truer rendition ofblack portions of the original may thus be achieved than can be obtainedby a combination of the primary colors. As another example, Edwin H.Land suggests in "The Retinex Theory of Color Vision", ScientificAmerican, pages 108-128 (December, 1977) and "Color Vision and theNatural Image", Proceedings of the National Academy of Science, pages115-129 and 636-644 (volume 45, 1959) that only two colors (and,therefore, only two laser diodes) need be mixed to produce a broad andcontinuous range of colors.

Various modifications and alterations of this invention will be apparentto those skilled in the art and it is intended that this inventioninclude all such modifications and alterations which fall within thescope and spirit of the appended claims.

We claim:
 1. A laser diode imaging apparatus capable of providing a fullcolor, continuous tone image without exposure to radiation within thevisible region of the electromagnetic spectrum comprising:(a) aphotographic element including:(1) a substrate, and (2) on one side ofsaid substrate at least three silver halide emulsion layers, each ofsaid silver halide emulsion layers being associated with a differentcolor photographic coupler, each of said couplers being capable offorming a different color dye upon reaction with an oxidized colorphotographic developer, said three silver halide emulsion layerscomprising, in order from the substrate to the surface of saidphotographic element, a first emulsion sensitized to a portion of theinfrared region of the electromagnetic spectrum, a second emulsionsensitized to a portion of the infrared region of the electromagneticspectrum which is of a shorter wavelength than the portion to which saidfirst emulsion is sensitized, and a third emulsion sensitized to aportion of the infrared region of the electromagnetic spectrum which isof a shorter wavelength than the portion to which said second emulsionis sensitized, and said three silver halide emulsion layers having aconstruction selected from the group consisting of:(i) each of the threelayers having a contrast between 0.5 and 12 and differing from eachother in photographic speed such that, at an optical density of 1.3, thespeed of the third emulsion is at least 0.2 log E units faster than thesecond emulsion layer, and the second emulsion is at least 0.2 log Eunits faster than the first emulsion layer, (ii) between said first andsecond emulsion layers is a filter layer absorbing infrared radiation ina range overlapping the region of maximum sensitivity of said secondemulsion layer without absorbing more than forty percent of the infraredradiation to which said first emulsion layer is sensitized, and betweensaid second emulsion layer and said third emulsion layer is a filterlayer absorbing radiation in a range overlapping the region of maximumsensitivity of said third emulsion layer without absorbing more thanforty percent of the infrared radiation to which said second layer issensitized, and (iii) directly between two layers comprising either saidfirst and second emulsion layers or said second and third emulsionlayers a filter layer absorbing radiation in a range overlapping theregion of maximum sensitivity the one of the two layers further awayfrom the substrate without absorbing more than forty percent of theinfrared radiation to which the other of said two layers is sensitizedand the other pair of emulsion layers comprising said second and thirdemulsion layers and said first and second emulsion layers, respectively,having a contrast between 0.5 and 12 and differing in speed from eachother so that at an optical density of 1.3, the speed of the emulsionlayer farthest from the substrate in said other pair of emulsion layersis at least 0.02 log E units faster than the speed of the emulsion layerclosest to the substrate in said other pair of emulsion layers; and (b)three laser diodes for exposing said photographic element, each laserdiode emitting radiation in the infrared region of the electromagneticspectrum and at a particular wavelength which is different from that ofany of the other laser diodes and which corresponds to a different oneof those portions of the electromagnetic spectrum at which said first,said second and said third emulsion layers are sensitized.
 2. The laserdiode imaging apparatus of claim 1 further including means formodulating continuously the intensity of said radiation emitted by saidlaser diodes to correspondingly modulate said exposure of saidphotographic elements and thereby the density of said color upondevelopment.
 3. The laser diode imaging apparatus of claim 1 furtherincluding means for directing said radiation emitted by said laserdiodes to said photographic element, focusing said radiation to a spotand transversely scanning said radiation spot across the surface of saidphotographic element.
 4. The laser diode imaging apparatus of claim 1further including means for advancing said photographic element relativeto said scanned radiation in a direction having a component directionperpendicular to said direction of scanning so that all portions of saidphotographic element are exposed to said radiation.
 5. The laser diodeimaging apparatus of claim 1 in which the contrast of each of said atleast three silver halide emulsion layers is between 2 and
 8. 6. Thelaser diode imaging apparatus of claim 5 in which the construction hasat least one filter layer between a pair of adjacent emulsion layerswhich absorbs between ten and eighty percent of the infrared radiationto which the layer farther from the substrate is sensitized whileabsorbing less than forty percent of the infrared radiation to which thelayer closer to the substrate is sensitized.
 7. The laser diode imagingapparatus of claim 5 in which two filter layers are present, one betweensaid first and a second emulsion layer and one between said second andthird emulsion layer, each of said filter layers absorbing at least tenand less than eighty percent of the infrared radiation to which theadjacent layer farther from the substrate is sensitized while absorbingless than twenty-five percent of the infrared radiation to which theadjacent layer closer to the substrate is sensitized.
 8. The laser diodeimaging apparatus of claim 5 in which at least two adjacent emulsionlayers differ in their photographic speed and have a contrast between 2and 5, the speed difference between said two adjacent layers being suchthat at an optical density of 1.3 the speed of the adjacent emulsionlayer closest to the substrate is at least 0.05 log E units slower thanthe speed of the adjacent emulsion layer farthest from the substrate. 9.The laser diode imaging apparatus of claim 5 in which both pairs ofadjacent emulsion layers in a three emulsion layer system differ intheir photographic speed and have a contrast between 2 and 5, the speeddifference between adjacent layers being such that at an optical densityof 1.3 the speed of the adjacent emulsion layer of each pair closest tothe substrate is at least 0.5 log E units slower than the speed of theadjacent emulsion layer farther from the substrate.
 10. A laser diodeimaging apparatus capable of providing a full color image with exposureof a photosensitive element having at least two silver halide emulsionlayers sensitized to radiation within the infrared region of theelectromagnetic spectrum comprising:(a) a photosensitive elementhaving:(1) a substrate, and (2) on one side of said substrate at leastthree silver halide emulsion layers, each of said silver halide emulsionlayers being associated with a means for providing a different color dyeimage, said three silver halide emulsion layers comprising, in ordertowards the surface of said photographic element to be exposed, a firstemulsion sensitized to a portion of the infrared region of theelectromagnetic spectrum, a second emulsion sensitized to a portion ofthe infrared region of the electromagnetic spectrum which is of ashorter wavelength than the portion to which said first emulsion issensitized, and a third emulsion sensitized to a portion of theelectromagnetic spectrum which is of a shorter wavelength than theportion to which said second emulsion is sensitized, and said threesilver halide emulsion layers having a construction selected from thegroup consisting of:(i) each of the three layers having a contrastbetween 0.5 and 12 and the first two layers differing from each other inphotographic speed such that, at an optical density of 1.3, the speed ofthe second emulsion layer is at least 0.2 log E units faster than thefirst emulsion layer, and (ii) between said first and second emulsionlayers is a filter layer absorbing infrared radiation in a rangeoverlapping the region of maximum sensitivity of said second emulsionlayer without absorbing more than forty percent of the infraredradiation to which said first emulsion layer is sensitized. (b) threelaser diodes for exposing said photosensitive element, each laser diodeemitting radiation in the region of the electromagnetic spectrum and ata particular wavelength which is different from that of any of the otherlaser diodes and which corresponds to a different one of those portionsof the electromagnetic spectrum at which said first, said second andsaid third emulsion layers are sensitized.
 11. The laser diode imagingapparatus of claim 10 further including means for modulatingcontinuously the intensity of said radiation emitted by said laserdiodes to correspondingly modulate said exposure of said photographicelements and thereby the density of said color upon development.
 12. Thelaser diode imaging apparatus of claim 10 further including means fordirecting said radiation emitted by said laser diodes to saidphotographic element, focusing said radiation to a spot and transverselyscanning said radiation spot across the surface of said photographicelement.
 13. The laser diode imaging apparatus of claim 10 furtherincluding means for advancing said photographic element relative to saidscanned radiation in a direction having a component directionperpendicular to said direction of scanning so that all portions of saidphotographic element are exposed to said radiation.
 14. The laser diodeimaging apparatus of claim 10 in which the contrast of each of said atleast three silver halide emulsion layers has a contrast between 2 and8.
 15. The laser diode imaging apparatus of claim 10 in which theconstruction has at least one filter layer between a pair of adjacentemulsion layers which absorbs between ten and eighty percent of theinfrared radiation to which the layer farther from the substrate issensitized while absorbing less than forty percent of the infraredradiation to which the layer closer to the substrate is sensitized. 16.The laser diode imaging apparatus of claim 10 in which two filter layersare present, one between said first and a second emulsion layer and onebetween said second and third emulsion layer, each of said filter layersabsorbing at least ten and less than eighty percent of the radiation towhich the adjacent layer farther from the substrate is most stronglysensitized while absorbing less than twenty-five percent of the infraredradiation to which the adjacent layer closer to the substrate issensitized.
 17. The laser diode imaging apparatus of claim 10 in whichat least two adjacent emulsion layers differ in their photographic speedand have a contrast between 2 and 5, the speed difference between saidtwo adjacent layers being such that at an optical density of 1.3 thespeed of the adjacent emulsion layer closest to the substrate is atleast 0.05 log E units slower than the speed of the adjacent emulsionlayer farther from the substrate.
 18. The laser diode imaging apparatusof claim 10 in which both pairs of adjacent emulsion layers in a threeemulsion layer system differ in their photographic speed and have acontrast between 2 and 5, the speed difference between adjacent layersbeing such that at an optical density of 1.3 the speed of the adjacentemulsion layer of each pair closest to the substrate is at least 0.5 logE units slower than the speed of the adjacent emulsion layer fartherfrom the substrate.
 19. The laser diode imaging apparatus of claim 10 inwhich said means of providing a different color comprises a dye-transferprocess.
 20. The laser diode imaging apparatus of claim 10 in which saidmeans of providing a different color comprises a dye-bleach process. 21.The laser diode imaging apparatus of claim 10 in which said means ofproviding a different color comprises a leuco dye oxidation process. 22.The laser diode imaging apparatus of claim 10 in which said means ofproviding a different color comprises the reaction between aphotographic color coupler in each emulsion layer with an oxidized colorphotographic developer.
 23. A laser diode imaging apparatus capable ofproviding a full color image exposure of a photosensitive element havingat least two silver halide emulsion layers sensitized to radiationwithin the infrared region of the electromagnetic spectrumcomprising:(a) a photosensitive element including:(1) a substrate, and(2) on one side of said substrate at least three silver halide emulsionlayers, each of said silver halide emulsion layers being associated withthe means for providing a different color dye image, said three silverhalide emulsion layers comprising, a first emulsion sensitized to aportion of the infrared region of the electromagnetic spectrum, a secondemulsion sensitized to a portion of the infrared region of theelectromagnetic spectrum which is of a shorter wavelength than theportion to which said first emulsion is sensitized, and a third emulsionsensitized to a portion of the electromagnetic spectrum which is of ashorter wavelength than the portion to which said second emulsion issensitized, and said three silver halide emulsion layers having aconstruction selected from the group consisting of:(i) each of the threelayers having a contrast between 0.5 and 12 and the first two layersdiffering from each other in photographic speed such that, at an opticaldensity of 1.3, the speed of the second emulsion layers, is at least 0.2log E units faster than the first emulsion layer, and (ii) between saidfirst and second emulsion layers is a filter layer absorbing infraredradiation in a range overlapping the region of maximum sensitivity ofsaid second emulsion layer without absorbing more than forty percent ofthe infrared radiation to which said first emulsion layer is sensitized;and (b) three laser diodes for exposing said photosensitive element,each laser diode emitting radiation in the region of the electromagneticspectrum and at a particular wavelength which is different from that ofany of the other laser diodes and which corresponds to a different oneof those portions of the electromagnetic spectrum at which said first,said second and said third emulsion layers are sensitized.
 24. The laserdiode imaging apparatus of claim 23 further including means formodulating continuously the intensity of said radiation emitted by saidlaser diodes to correspondingly modulate said exposure of saidphotographic elements and thereby the density of said color upondevelopment.
 25. The laser diode imaging apparatus of claim 23 furtherincluding means for directing said radiation emitted by said laserdiodes to said photographic element, focusing said radiation to a spotand transversely scanning said radiation spot across the surface of saidphotographic element.
 26. The laser diode imaging apparatus of claim 23further including means for advancing said photographic element relativeto said scanned radiation in a direction having a component directionperpendicular to said direction of scanning so that all portions of saidphotographic element are exposed to said radiation.
 27. The laser diodeimaging apparatus of claim 23 in which the contrast of each of said atleast three silver halide emulsion layers has a contrast between 2 and8.
 28. The laser diode imaging apparatus of claim 27 in which said firstand second emulsion layers differ in their photographic speed and have acontrast between 2 and 5, the speed difference between said two adjacentlayers being such that at an optical density of 1.3 the speed of theadjacent emulsion layer closest to the substrate is at least 0.5 log Eunits slower than the speed of the adjacent emulsion layer farther fromthe substrate.
 29. The laser diode imaging apparatus of claim 27 inwhich both pairs of adjacent emulsion layers in a three emulsion layersystem differ in their photographic speed and have a contrast between 2and 5, the speed difference between adjacent layers being such that atan optical density of 0.13 the speed of the adjacent emulsion layer ofeach pair closest to the substrate is at least 0.5 log E units slowerthan the speed of the adjacent emulsion layer farther from thesubstrate.
 30. The laser diode imaging apparatus of claim 27 whereinsaid third emulsion layer is spectrally sensitized to a wavelengthwithin the visible portion of the electromagnetic spectrum and saidthird emulsion layer is further from the substrate than said first andsecond emulsion layers.
 31. The laser diode imaging apparatus of claim27 wherein said third emulsion layer is spectrally sensitized to awavelength within the visible portion of the electromagnetic spectrumand said third emulsion layer is located between said first and secondemulsion layers.
 32. The laser diode imaging apparatus of claim 27wherein said third emulsion layer is spectrally sensitized to awavelength within the visible portion of the electromagnetic spectrumand said third emulsion layer is closer to said substrate than saidfirst and second emulsion layers.
 33. A laser diode imaging apparatuscomprising a photographic element having at least three silver halideemulsion layers on a substrate, each of said three silver halideemulsion layers being capable of forming a single color image of adifferent color dye, said three silver halide emulsion layerscomprising, in any order, a first silver halide emulsion layersensitized to a portion of the infrared region of the electromagneticspectrum, a second silver halide emulsion layer sensitized to adifferent portion of the infrared region of the electromagneticspectrum, the wavelengths of maximum spectral sensitivity for said firstand second layer differing by at least 15 nm, and a third silver halideemulsion layer sensitized to a third portion of the electromagneticspectrum, the wavelength of maximum spectral sensitivity for said thirdlayer differing by at least 15 nm from the wavelength of maximumspectral sensitivity of said first and second layers, the sensitivity ofeach of said three silver halide emulsion layers being such that betweenany two layers having their maximum sensitivity in the infrared, theemulsion layer having the shorter wavelength of maximum spectralsensitivity has a speed which is at least 0.2 log E units faster thanthe other of said any two layers, and three laser diodes for exposingsaid photosensitive element, each laser diode emitting radiation in theregion of the electromagnetic spectrum and at a particular wavelengthwhich is different from that of any of the other laser diodes and whichcorresponds to a different one of said wavelengths of maximum spectralsensitivity of said first, said second and said third emulsion layers.34. The laser diode imaging apparatus of claim 33 further includingmeans for modulating continuously the intensity of said radiationemitted by said laser diodes to correspondingly modulate said exposureof said photographic elements and thereby the density of said color upondevelopment.
 35. The laser diode imaging apparatus of claim 33 furtherincluding means for directing said radiation emitted by said laserdiodes to said photographic element, focusing said radiation to a spotand transversely scanning said radiation spot across the surface of saidphotographic element.
 36. The laser diode imaging apparatus of claim 33further including means for advancing said photographic element relativeto said scanned radiation in a direction having a component directionperpendicular to said direction of scanning so that all portions of saidphotographic element are exposed to said radiation.
 37. The laser diodeimaging apparatus of claim 33 in which the contrast of each of said atleast three silver halide emulsion layers is between 0.5 and
 12. 38. Thelaser diode imaging apparatus of claim 37 wherein the wavelengths ofmaximum sensitivity for each of said at least three emulsion layersdiffer from each other by at least 35 nm and the contrast of each ofsaid three emulsion layers is from 1 to
 11. 39. The laser diode imagingapparatus of claim 38 wherein between said any two layers, the emulsionlayer having the shorter wavelength of maximum sensitivity has a speedwhich is at least 0.5 log E units faster than the other of said any twolayers.
 40. The laser diode imaging apparatus of claim 39 wherein saidmeans for providing a different color dye image is a photographic colorcoupler.
 41. The laser diode imaging apparatus of claim 39 wherein saidmeans for providing a different color dye image is diffusion transfer.42. The laser diode imaging apparatus of claim 33 in which the contrastof each of said at least three silver halide emulsion layers has acontrast between 2 and
 8. 43. The laser diode imaging apparatus of claim42 wherein the wavelengths of maximum sensitivity for each of said atleast three emulsion layers differ from each other by at least 50 nm andthe contrast of each of said three emulsion layers is from 2 to
 8. 44.The laser diode imaging apparatus of claim 43 wherein between said anytwo layers the emulsion layer having the shorter wavelength of maximumsensitivity has a speed which is at least 0.5 log E units faster thanthe other of said any two layers.
 45. The laser diode imaging apparatusof claim 42 wherein said means for providing a different color dye imageis a photographic color coupler.
 46. The laser diode imaging apparatusof claim 42 wherein said means for providing a different color dye imageis diffusion transfer.
 47. The laser diode imaging apparatus of claim 33wherein between said any two layers, the emulsion layer having theshorter wavelength of maximum sensitivity has a speed which is at least0.5 log E units faster than the other of said any two layers.
 48. Thelaser diode imaging apparatus of claim 33 wherein said means forproviding a different color dye image is a photographic color coupler.49. The laser diode imaging apparatus of claim 33 wherein said means forproviding a different color dye image is diffusion transfer.
 50. A laserdiode imaging apparatus capable of providing a full color, continuoustone image without exposure to radiation within the visible region ofthe electromagnetic spectrum comprising:(a) a photographic elementincluding:(1) a substrate, and (2) on one side of said substrate atleast three silver halide emulsion layers, each of said silver halideemulsion layers being associated with a different color photographiccoupler, each of said couplers being capable of forming a differentcolor dye upon reaction with an oxidized color photographic developer,said three silver halide emulsion layers comprising, in order from thesubstrate to the surface of said photographic element, a first emulsionsensitized to a portion of the infrared region of the electromagneticspectrum, a second emulsion sensitized to a portion of the infraredregion of the electromagnetic spectrum which is of a shorter wavelengththan the portion to which said first emulsion is sensitized, and a thirdemulsion sensitized to a portion of the infrared region of theelectromagnetic spectrum which is of a shorter wavelength than theportion to which said second emulsion is sensitized, and said threesilver halide emulsion layers having a construction comprising:(i) eachof the three layers having a contrast between 0.5 and 12 and differingfrom each other in photographic speed such that, at an optical densityof 1.3, the speed of the third emulsion is at least 0.2 log E unitsfaster than the second emulsion layer, and the second emulsion is atleast 0.2 log E units faster than the first emulsion layer, and (b)three laser diodes for exposing said photographic element, each laserdiode emitting radiation in the infrared region of the electromagneticspectrum and at a particular wavelength which is different from that ofany of the other laser diodes and which corresponds to a different oneof those portions of the electromagnetic spectrum at which said first,said second and said third emulsion layers are sensitized.
 51. The laserdiode imaging apparatus of claim 50 further including means formodulating continuously the intensity of said radiation emitted by saidlaser diodes to correspondingly modulate said exposure of saidphotographic elements and thereby the density of said color upondevelopment.
 52. The laser diode imaging apparatus of claim 50 furtherincluding means for directing said radiation emitted by said laserdiodes to said photographic element, focusing said radiation to a spotand transversely scanning said radiation spot across the surface of saidphotographic element.
 53. The laser diode imaging apparatus of claim 50further including means for advancing said photographic element relativeto said scanned radiation in a direction having a component directionperpendicular to said direction of scanning so that all portions of saidphotographic element are exposed to said radiation.
 54. The laser diodeimaging apparatus of claim 50 in which at least two adjacent emulsionlayers differ in their photographic speed and have a contrast between 2and 5, the speed difference between said two adjacent layers being suchthat an optical density of 1.3 the speed of the adjacent emulsion layerclosest to the substrate is at least 0.05 log E units slower than thespeed of the adjacent emulsion layer farthest from the substrate.
 55. Alaser diode imaging apparatus capable of providing a full color imagewith exposure of a photosensitive element having at least two silverhalide emulsion layers sensitized to radiation within the infraredregion of the electromagnetic spectrum comprising:(a) a photosensitiveelement having:(1) a substrate, and (2) on one side of said substrate atleast three silver halide emulsion layers, each of said silver halideemulsion layers being associated with a means for providing a differentcolor dye image, said three silver halide emulsion layers comprising, inorder towards the surface of said photographic element to be exposed, afirst emulsion sensitized to a portion of the infrared region of theelectromagnetic spectrum, a second emulsion sensitized to a portion ofthe infrared region of the electromagnetic spectrum which is of ashorter wavelength than the portion to which said first emulsion issensitized, and a third emulsion sensitized to a portion of theelectromagnetic spectrum which is of a shorter wavelength than theportion to which said second emulsion is sensitized, and said threesilver halide emulsion layers having a construction selected from thegroup consisting of:(i) each of the three layers having a contrastbetween 0.5 and 12 and the first two layers differing from each other inphotographic speed such that, at an optical density of 1.3, the speed ofthe second emulsion layer, is at least 0.2 log E units faster than thefirst emulsion layer, and (b) three laser diodes for exposing saidphotosensitive element, each laser diode emitting radiation in theregion of the electromagnetic spectrum and at a particular wavelengthwhich is different from that of any of the other laser diodes and whichcorresponds to a different one of those portions of the electromagenticspectrum at which said first, said second and said third emulsion layersare sensitized.
 56. The laser diode imaging apparatus of claim 55further including means for modulating continuously the intensity ofsaid radiation emitted by said laser diodes to correspondingly modulatesaid exposure of said photographic elements and thereby the density ofsaid color upon development.
 57. The laser diode imaging apparatus ofclaim 55 further including means for directing radiation emitted by saidlaser diodes to said photographic element, focusing said radiation to aspot and transversely scanning said radiation spot across the surface ofsaid photographic element.
 58. The laser diode imaging apparatus ofclaim 55 further including means for advancing said photographic elementrelative to said scanned radiation in a direction having a componentdirection perpendicular to said direction of scanning so that allportions of said photographic element are exposed to said radiation. 59.The laser diode imaging apparatus of claim 55 in which the contrast ofeach of said at least three silver halide emulsion layers has a contrastbetween 2 and
 8. 60. The laser diode imaging apparatus of claim 55 inwhich at least two adjacent emulsion layers differ in their photographicspeed and have a contrast between 2 and 5, the speed difference betweensaid two adjacent layers being such that at an optical density of 1.3the speed of the adjacent emulsion layer closest to the substrate is atleast 0.05 log E units slower than the speed of the adjacent emulsionlayer farther from the substrate.
 61. A laser diode imaging apparatuscapable of providing a full color image exposure of a photosensitiveelement having at least two silver halide emulsion layers sensitized toradiation within the infrared region of the electromagnetic spectrumcomprising:(a) a photosensitive element including:(1) a substrate, and(2) on one side of said substrate at least three silver halide emulsionlayers, each of said silver halide emulsion layers being associated withthe means for providing a different color dye image, said three silverhalide emulsion layers comprising, a first emulsion sensitized to aportion of the infrared region of the electromagnetic spectrum, a secondemulsion sensitized to a portion of the infrared region of theelectromagnetic spectrum which is of a shorter wavelength than theportion to which said first emulsion is sensitized, and a third emulsionsensitized to a portion of the electromagnetic spectrum which is of ashorter wavelength than the portion to which said second emulsion issensitized, and said three silver halide emulsion layers having aconstruction wherein:(i) each of the three layers having a contrastbetween 0.5 and 12 and the first two layers differing from each other inphotographic speed such that, at an optical density of 1.3, the speed ofthe second emulsion layers is at least 0.2 log E units faster than thefirst emulsion layer, and (b) three laser diodes for exposing saidphotosensitive element, each laser diode emitting radiation in theregion of the electromagnetic spectrum and at a particular wavelengthwhich is different from that of any of the other laser diodes and whichcorresponds to a different one of those portions of the electromagneticspectrum at which said first, said second and said third emulsion layersare sensitized.
 62. The laser diode imaging apparatus of claim 61further including means for modulating continuously the intensity ofsaid radiation emitted by said laser diodes to correspondingly modulatesaid exposure of said photographic elements and thereby the density ofsaid color upon development.
 63. The laser diode imaging apparatus ofclaim 61 further including means for directing said radiation emitted bysaid laser diodes to said photographic element, focusing said radiationto a spot and tranversely scanning said radiation spot across thesurface of said photographic element.
 64. The laser diode imagingapparatus of claim 61 further including means for advancing saidphotographic element relative to said scanned radiation in a directionhaving a component direction perpendicular to said direction of scanningso all portions of said photographic element are exposed to saidradiation.
 65. The laser diode imaging apparatus of claim 61 in whichthe contrast of each of said at least three silver halide emulsionlayers has a contrast between 2 and
 8. 66. The laser diode imagingapparatus of claim 61 in which said first and second emulsion layersdiffer in their photographic speed and have a contrast between 2 and 5,the speed difference between said two adjacent layers being such that atan optical density of 1.3 the speed of the adjacent emulsion layerclosest to the substrate is at least 0.5 log E units slower than thespeed of the adjacent emulsion layer farther from the substrate.
 67. Thelaser diode imaging apparatus of claim 61 in which both pairs ofadjacent emulsion layers in a three emulsion layer system differ intheir photographic speed and have a contrast between 2 and 5, the speeddifference between adjacent layers being such that at an optical densityof 0.13 the speed of the adjacent emulsion layer of each pair closest tothe substrate is at least 0.5 log E units slower than the speed of theadjacent emulsion layer farther from the substrate.
 68. The laser diodeimaging apparatus of claim 61 wherein said third emulsion layer isspectrally sensitized to a wavelength within the visible portion of theelectromagnetic spectrum and said third emulsion layer is further fromthe substrate than said first and second emulsion layers.
 69. The laserdiode imaging apparatus of claim 61 wherein said third emulsion layer isspectrally sensitized to a wavelength within the visible portion of theelectromagnetic spectrum and said third emulsion layer is locatedbetween said first and second emulsion layers.
 70. The laser diodeimaging apparatus of claim 61 wherein said third emulsion layer isspectrally sensitized to a wavelength within the visible portion of theelectromagnetic spectrum and said third emulsion layer is closer to saidsubstrate than said first and second emulsion layers.